We are determining the internal structure of 40 nm chromosome fibers by the use of low angle x-ray scattering and electron microscopy. Comparisons of the diffraction patterns from nucleofibers in living cells, isolated nuclei and isolated fibers will enable us to identify structural alterations caused by various preparative procedures, including those used for electron microscopy. Analysis of diffraction patterns from oriented, intact fibers or of electron micrographs of well-preserved fibers will hopefully elucidate the mode of nucleosome packing within nucleofibers. By studying chromosome structure in different tissues and under different growth conditions we hope to understand more about the relationships between chromosome structure and function. Knowledge about the structure of inactive chromosomes and about the gross structural changes that take place during cell growth and differentiation, particularly during the G0/G1 transition, are potentially important to the understanding of cancer.